The present invention relates to a method of promptly, safely and exactly detecting a defect formed in such a structural part of an apparatus which handles a liquid metal as being invisible directly from the outside of the apparatus, e.g. a heat transfer tube nest of a sodium-heated steam generator, as well as to a device for carrying out the method.
Heat exchangers as generally referred to as intermediate heat exchanger or steam generator are used in a nuclear reactor of a type called fast breeder which makes use of, for example liquid sodium. A large number of heat transfer tubes are disposed in these heat exchangers, such that heat is exchanged between two fluids flowing inside and outside of the heat transfer tubes, across the wall of the latter.
Although the severest consideration has been taken in the selection of material, design and manufacture of these heat transfer tube, it is impossible to completely preclude the possibility of leaking accident.
When a leaking accident has taken place, it is most important and significant to thoroughly investigate the cause of the accident. Unless the cause of the leaking accident is clarified and removed, the accident will repeatedly take place, even if a stopgap measure, such as closing the ends of the defective tube by plugs, is taken. Thus, once the leaking accident takes place, it becomes necessary to suspend the operation of the plant for a long period of time, for clarifying and removing the cause of the accident, which considerably lowers the efficiency of work of the plant.
Usually, these heat exchangers are of a type so called shell and tube type, in which a heat transfer tube nest is removably installed in a shell, so as to facilitate the protective maintenance and repair. However, it is almost impossible to inspect these tubes by naked eyes, because of a specific condition of attaching of sodium and complicated arrangement of the heat transfer tubes. Thus, no effective method of inspection has been established yet for these specific conditions.
To explain in more detail, there have been proposed and used various methods of detecting defects during the use of the heat exchanger, e.g. ultrasonic wave method, eddy current method, acoustic emission method (AE method) and so forth. These conventional methods are, however, not effective enough for remote examination of heat transfer tubes for the following reasons. Namely, for the first point, it is difficult to insert the probe and handle the same in the heat exchanger. For the second point, it takes a considerably long time to scan the probe to examine the entire area of the heat exchanger in the circumferential and longitudinal directions. For the third point, the sensitivity of detection is considerably deteriorated, because of the signals reflected by support members disposed outside of the heat transfer tubes and by scales of sodium attaching to the heat transfer tubes, as well as non-directional signals emitted by welded metal parts.
It is therefore necessary to work out a novel method of defect detection which would afford inspection of complicated structural part of apparatus for use in liquid metal, well meeting the above stated specific condition of detection.
Accordingly, an object of the present invention is to provide a method of promptly and surely detecting the position of the defect in liquid metal handling instruments.
Another object of the present invention is to provide a method which allows a prompt and thorough investigation of the cause of the defect in liquid metal handling instruments.
Another object of the present invention is to provide a method of detecting the position of the defect in liquid metal handling instrument, which allows a repairing work in a minimum time.
Another object of the present invention is to provide an apparatus which permits a prompt and reliable investigation of the position of the defect in liquid metal handling instruments.
A further object of the present invention is to provide an apparatus which permits a thorough investigation of the cause of the defect in liquid metal handling instruments.